1. Field of the Invention
This invention relates generally to power supplies that detect and prevent or extinguish arcs in plasma processing applications, and more particularly to dc processes and apparatus and methods to extinguish arcs therein.
2. Brief Description of the Prior Art
A cathodic arc is said to develop in a plasma-based process when a discharge occurs between a point on an insulator where charge has accumulated and a point on the cathode. The impedance of the plasma then collapses and this is often termed a micro arc. In the past, these micro arcs were often ignored as not important. Oftentimes their very existence went undetected, since the power supply did not detect their existence directly. If a micro arc does not extinguish fast enough, there can be a cascading effect where the micro arc develops into what is termed as a hard arc. The plasma is then discharged through this hard arc, creating a very high density of power that can, if not extinguished quickly, be very detrimental to the process and the quality of the film.
Past approaches to arc control in dc plasma processes have focused upon the reduction of energy supplied by a dc power supply into an arc, or they have used high frequency or medium frequency power in conjunction with dc power to minimize or eliminate the onset of arcing. Power supplies that store very little energy, such as those taught in U.S. Pat. No. 5,535,906, deliver very little energy into an arc beyond what is stored in the output cable. Such a power supply extinguishes arcs by turning off after the arc is detected. After the power supply turns off, the energy stored in the cable is dissipated into the arc and losses in the cable and power supply. This type of power supply requires active circuitry to sense the arcs, and to turn the power supply on and off.
Alternatively, passive circuitry can be used to extinguish arcs as they occur without disturbing the power supply's regulation or logic circuits, as is taught in U.S. Pat. No. 5,645,698 and U.S. Pat. No. 6,524,455, for example. The passive elements generally include an inductance and capacitance to provide a resonant circuit that rings the current to zero in the plasma, thus turning off the arc. The inductance may include a discrete inductor, but may also be just the inductance of the output cable that connects the power supply to a plasma chamber. Typical times of this ring out range from a few microseconds to tens of microseconds. Passive ring out circuits produce large overshoots in the output current, and depending on the values of the resonant components, this may result in large amounts of energy being dumped into the process. U.S. Pat. No. 6,524,455 and JP 61030655A teach resonant ring out circuits that can extinguish arcs in a few microseconds.
In many power supplies, the output filter inductance stores much more energy than the cable inductance, so if the power supply is simply turned off after an arc is sensed, a large amount of energy may be delivered to the arc even when the energy stored in the output capacitance is relatively low. One approach to reduce the amount of the stored inductive energy that is delivered to the load uses a switch in series with the output of the power supply that is opened so as to prevent the current from reaching the plasma. Because this method involves interruption of inductor current, large voltage spikes can be developed across the switch, and this requires having an effective means of dissipating the inductive energy to protect the switch.
It is generally accepted in the industry that the best approach to extinguish arcs is to reduce the current through them to zero, or some predetermined low threshold value. FIG. 1 illustrates the typical arc ring out process. The easiest way to ring out the arc is to create a resonant circuit, not shown, between the capacitor Cout on the output of the power supply which also typically serves as the filter capacitor for the dc power supply and an inductance Larcout that may include a discrete inductor inserted in series with the output of the power supply, but may also be just the inductance of the output cable of the power supply. In typical ring out circuits, the arc is extinguished by driving the current to zero.
As shown in FIG. 1, the time between t1 to t2 is the initiation of the arc. The ring out occurs between times t2 and t3. The current reaches zero at time t3, and the arc is extinguished. At t4, the plasma recovers naturally. Ipeak is the peak value of the current produced by the ring out. The voltage waveforms plotted in FIGS. 1 and 3 are the voltage at the chamber, referenced to the negative input terminal of the chamber. The current waveforms illustrate the current flowing into the positive input terminal of the chamber.
The approximate values of Ipeak, and the time interval ting as defined in FIG. 1, can be calculated by the following equations:                                                                         t                ring                            =                            ⁢                              π                ·                                                                            L                      arcout                                        ·                                          C                      out                                                                                                                                                              I                peak                            =                            ⁢                                                V                  dc                                /                                  Z                  arcout                                                                                                      where          ⁢                                          ⁢                      Z            arcout                          =                                                            L                arcout                            /                              C                out                                              .                    The passive ring out approach has at least the following disadvantages:    1) In order to ensure that the current consistently reaches zero, the peak current has to be at least twice the dc value of the current. This means that the current peaks can be very high.    2) In order to ensure that the current rings to zero, the value of Zarcout must be low enough to accommodate the maximum output current at the minimum output voltage. This causes the ring out currents to be considerably greater than is necessary when operating at output voltages that are higher than the minimum output voltage.    3) The power supply output voltage reverses polarity as the output current is driven toward zero. Having too large of a reverse voltage can be detrimental if a reverse current flows after the arc, so extra circuitry to prevent reverse currents may sometimes be required, as is taught in U.S. Pat. No. 6,645,698 and U.S. Pat. No. 6,524,455.
It would be desirable if there were provided a power supply for dc plasma processes that prevented micro arcs from developing into hard arcs and allowed a fast ring out of arcs. It would also be desirable if power supplies provided an ultra-low energy discharge into arcs, and which also operated sufficiently fast to prevent the plasma from being extinguished in sensitive applications such as self-ionized plasmas. It is an object of this invention to provide a power supply and method for a dc based plasma process that diverts current from the plasma at the initiation of an arc.